Conventionally, high-frequency power amplifiers are often operated in the AB-mode, that is to say, a base bias voltage is generated by means of a bias configuration, for example, in the form of series-connected diodes, which even at low levels drive the transistors used to full output. In the AB mode, small signals, such as in the A mode, and large signals, such as in the B mode, are amplified. Such amplifiers operate with good efficiency with a particularly low-distortion, but only if they are operated with full level control close to compression. In the case of modern digital signals with strongly fluctuating envelope, this is only the case with the very rarely occurring signal peaks.
Since the mean level control with these signals is low, a relatively low average efficiency is obtained. With suitable dimensioning, AB amplifiers with a very broad bandwidth can be realized.
Furthermore, to improve the efficiency of such signals, Doherty amplifiers are known. In mobile-radio technology, especially in the case of modulation methods with non-constant envelope and high crest factor (for example COFDM signals), high-frequency amplifiers according to the Doherty principle have become increasingly widespread over recent years. For example, the German Patent Application DE 10 2010 034 067 A1 discloses such a Doherty amplifier. However, the disadvantage with such amplifiers is that they provide only a very narrow bandwidth.
In conventional transmitter systems, several Doherty amplifiers are used. Since these are embodied in each case with a narrow bandwidth, such transmitter systems provide a redundant transmitter, which can take over the function of a failed transmitter. However, so that the redundant transmitter can take over the function of every Doherty transmitter independently of its frequency range, this redundant transmitter is conventionally designed with a broad bandwidth. Accordingly, in an exemplary transmitter system with three different Doherty frequencies, transmitters with at least four different amplifier arrangements are installed. This incurs a high cost for keeping corresponding replacements in reserve.